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Old 29th Apr 2004, 9:36 pm   #1
parangles
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Default Help with using sig. gen with transistors?

Hello to all,
I have finally got my generator working and want to use it to troubleshoot/diagnose transistor radios.
I have numerous repair books and have downloaded 3 manuals for signal generators but they are all refer to valve radios. Eg. They say " Inject a signal at the IF " . Attach which leg of the output lead/cable where? and the other one where? Which leg of the transistor? or elsewhere? I want to trace backways from the speaker.
I understand the RF vs AF and adding modulation etc.
Thanks !
Chris
 
Old 29th Apr 2004, 10:42 pm   #2
ukcol
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Default Re: Help with using sig. gen with transistors?

Hi Chris

The input electrode of a transistor is the base which is equivelent to the control grid of a valve. The screen of the coax cable from the signal generator needs to be connected to chassis and the inner wire to the base of the transistor via a capacitor and a resistor in series. The capacitor is to block DC so you don't upset the bias conditions of the transistor and the resistor is to prevent accidental damage to the transistor due to excess base current. Suitable values are 0.1uF and 2.2k ohms respectively.

Colin
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Old 29th Apr 2004, 10:47 pm   #3
jim_beacon
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Default Re: Help with using sig. gen with transistors?

Chris,

normally you would inject a signal at the IF frequency into the base of the mixer transistor, usually via a DC blocking capacitor (0.01uF seems a popular choice). The " hot " or signal lead is connected via the capacitor, the ground lead goes to the ground rail of the set.

When injecting signals in this way, you normally use a modulated signal, and either listen to the ouput level from the 'speaker, or connect an ouput meter across the 'speaker (high impedance meter), or in place of the speaker (meter impedance = 'speaker impedance). A 20K Ohm/volt multi-meter can be used as an ouput meter, but remeber it will not react well to high modulating frequencies (should be OK at 400Hz).

When aligning the set, keep the signal generator level as low as possible - if you use a high level, the AGC (AVC) action will mask any changes you make to the alignment. To find the level at which the AGC starts to work, start with a very low input signal, and watching the output meter, increase the input signall until the rate of change of ouput becomes much less than that of the input. Ideally, once the AGC begins to work, a large change of input signal level should not alter the output level, but it is seldom this good (3dB ouput change for 20dB input change is more like it). You will have to keep reducing the input level as you align the set, to prevent problems with the AGC.

Hope that helps.

Jim.
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Old 30th Apr 2004, 12:27 am   #4
parangles
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Default Re: Help with using sig. gen with transistors?

The circuit/schematic shows a .01 mfd directly behind the RF terminal output. This is a slightly more recent generator ( late 60's, early 70's maybe) sold by Radio Shack ( I think the eqv. of your ? Maplins stores )
Do I need to add another one ?
Tried to copy & paste a small .bmp section of it here but nogo.
I built the little Milliamp meter from this site to use across the speakers.
Will get back soon.
You're going to get tired of me on this site !
Thanks,
Chris.
 
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